An internal combustion engine includes a cylinder inner pressure sensor for outputting a signal in accordance with an inner pressure of a combustion chamber of a cylinder of the engine. A body of the cylinder inner pressure sensor is arranged in an insertion hole formed on a cylinder head. The pressure in the combustion chamber is applied to a Piezo electric device via a diaphragm of the sensor. An output voltage of the Piezo electric device is output through an amplifier circuit in the sensor. A sensor signal from the sensor is input into an engine ECU (electronic control unit), and the engine ECU converts the signal to a digital signal so that an A/D conversion process is performed. Then, the engine ECU detects the inner pressure of the cylinder based on the converted signal. Further, the engine ECU detects a firing state of the engine such as an ignition timing and firing temperature based on the inner pressure. Thus, the engine ECU can detect a knocking of the engine and a misfire of the engine.
However, under a condition that the temperature and/or the pressure of the engine are rapidly and largely changed, the output voltage of the cylinder inner pressure sensor drifts due to the variation of the temperature and the pressure. When the voltage of the sensor signal, i.e., the offset voltage, is measured around at a suction stroke, at which the cylinder inner pressure is minimum, the offset voltage varies according to the combustion state.
A means for preventing reduction of detection accuracy caused by the offset drift of the cylinder inner pressure sensor is to compensate the offset drift according to the processing of data obtained by converting the sensor signal with an A/D conversion method. This means is described in JP-A-2002-242750 and JP-A-2010-196556. Alternatively, the means is to cancel the offset drift by resetting the cylinder inner pressure sensor and/or an amplifier in an electric device at a certain time. This means is described in JP-A-H07-280686 and JP-A-2008-216223.
When the means described in JP-A-2002-242750 and JP-A-2010-196556 is used, the offset drift of the sensor signal output from the cylinder inner pressure sensor is not reduced. Thus, the offset drift of the sensor signal exists. Accordingly, it is necessary to set the gain of the cylinder inner pressure sensor and/or the input circuit to be low in order to always keep the sensor signal within the input range of the A/D converter circuit even when the offset voltage changes. Further, when the means described in JP-A-H07-280686 and JP-A-2008-216223, the offset voltage is reduced just after the sensor and/or the amplifier are reset. However, the offset drift occurs before the sensor and/or the amplifier are reset. Accordingly, it is necessary to set the gain of the cylinder inner pressure sensor and/or the input circuit to be low in order to always keep the sensor signal within the input range of the A/D converter circuit even when the offset voltage changes.
An offset voltage of a low price cylinder inner pressure sensor may change from −0.5 volts to +0.5 volts when the input range is 5 volts. When the gain is set to be low so that the sensor signal is disposed in the input range even if the offset voltage changes in a wide range, the dynamic range of the A/D converter circuit becomes narrow.